Actually, that's not always true. Large birds, for example, on the Pacific Islands have not gone extinct faster than small birds. But I imagine the questioner was thinking of, for example, the dinosaurs or the large mammals that were extinct more recently. And the general reasons that make them extinction prone is, by virtue of their size, they have longer generation rates; often they have lower reproductive rates per generation; and there are fewer of them -- their populations are smaller.

So all of those forces tend to make them more extinction-prone. It's interesting that, in spite of that, in some groups like island birds, there are other forces that counterbalance that. But the general reasons are exactly those: They have smaller population sizes, longer generational times, and lower rates of reproduction.

Ariel Lugo

The other reason is that, if you're being eaten by a predator, you have more biomass per unit catch effect. And if you're bigger, it's harder for you to hide. So all those things lead to extinction. But I agree that small organisms, they are highly affected by some of the habitat modification that we do. You just never know what you're doing, because you don't see them.

Tundi Agardi

It tends to be that the bigger species have more parental care and, as Dan said, they tend to be either slower growing or have a longer life or have a low reproductive rate. So I think species that fit that pattern tend to be more vulnerable to extinction because of those kinds of life history characteristics.